It's the daycare. Sorry. They are dirty, no matter what, and there are thousands of different stomach bugs you could be getting each time. If workers there are changing diapers and not washing their hands, there are poop germs (e Coli, salmonella) everywhere, in addition to all the viruses out there that are usually going around this time of year.
You should take lots of Vitamin C (not when you are sick to your stomach, of course!) and you should also take probiotics (Culterelle is a good one). You should take fish oil, also, and make sure you have a good diet full of plants and fruits and LOTS of water. Sleep is vital, also.
Dramatically reducing sugar can also help because sugar suppresses your immune system.
Make sure everyone in your home is washing their hands after going potty, too. EVERYONE.
The good news, is, eventually you will all become immune to these nasty bugs. You have to get them or at least be exposed to them to get immunity, though.